The present invention relates to an agent for stabilizing dispersion (hereinafter referred to as "dispersing stabilizer") in suspension polymerization of vinyl chloride monomer alone or a mixture containing a major amount of the monomer, and a process for preparing the same. More particularly, the invention relates to a dispersing stabilizer for homo- or co-polymerization of vinyl chloride, which enables to produce a vinyl chloride resin having good physical properties such as a large porosity, a large bulk density and a uniform distribution of particle size with a less amount of coarse particles. Further, the invention relates to a process for the suspension polymerization of vinyl chloride or its mixture in the presence of the stabilizer, particularly the process which is improved in producibility by adopting a hot water charging polymerization manner to raise the efficiency of polymerization cycle.
In a suspension polymerization of vinyl chloride or a mixture of vinyl chloride and other monomers copolymerizable therewith, it is necessary to employ one or more of various dispersing stabilizers, e.g. polyvinyl alcohol, methylolcellulose, vinyl acetate/maleic anhydride copolymer, and gelatin. Above all, polyvinyl alcohols have excellent properties as the dispersing stabilizer, and are the most generally employed. While there are various polyvinyl alcohols, it is known that the physical or chemical characteristics of polyvinyl alcohols, such as average degree of polymerization and average degree of hydrolysis, have a delicate influence upon the produced vinyl chloride resins. So, a number of proposals have been made for the purpose, for example, (a) a polyvinyl alcohol defined by degree of hydrolysis, degree of polymerization, content of carbonyl group and content of vinylene group within specified ranges (Japanese Unexamined Patent Publication Kokai No. 45189/1976), and (b) a polyvinyl alcohol modified by maleic acid and/or an ester of maleic acid (Japanese Examined Patent Publication Kokoku No. 59242/1982).
In general, the suspension polymerization of vinyl chloride has been carried out in a batchwise operation, wherein a polymerization vessel is charged with a water medium, a dispersing stabilizer, a polymerization initiator, vinyl chloride or its mixture, and optionally other necessary additives, followed by gradually raising the temperature of the mixture up to the polymerization temperature to carry out the polymerization
Such a batchwise operation, however, requires a very long time to feed the ingredients and to raise the temperature, which may sometimes account for 10% or more of the polymerization time in one cycle. It is a cause of decrease in productivity of the polymerization, which is a problem long wanted to be solved in the art. In order to solve the problem, various processes have been proposed. Recently, there is proposed a "hot water charging polymerization process", in which a hot water heated at 50.degree. C. or higher temperature is employed. This process has great effects in points of process steps for polymerization and shortening of time.
However, when suspension polymerization is carried out in such a hot water charging manner employing the above-mentioned polyvinyl alcohol (a) or (b) as the dispersing stabilizer, on feeding hot water, the polyvinyl alcohol is partly deposited out to make the aqueous solution turbid. Thus, not only cannot the polyvinyl alcohol produce its excellent effects as the stabilizer, but also it often affect adversely the polymerization stability and physical properties of the produced polymers.
Accordingly, a dispersing stabilizer, suitable for use in the hot water charging polymerization process and always capable of yielding vinyl chloride resins having good qualities, has been strongly demanded, since it would enable to advantageously carry out the suspension polymerization.
It is an object of the present invention to provide a dispersing stabilizer suitable for suspension homo- or co-polymerization of vinyl chloride, which can be used in the hot water charging polymerization process without causing any problem and give vinyl chloride resins of high quality.
A further object of the invention is to provide a process for preparing the dispersing stabilizer for suspension polymerization of vinyl chloride.
A still further object of the invention is to provide a process for the suspension homo- or copolymerization of vinyl chloride which can stably produce vinyl chloride resins having a high porosity, a high bulk density and a uniform distribution of particle size in a shortened period of time.
Another object of the invention is to provide a process for suspension-polymerizing vinyl chloride or its mixture with other copolymerizable monomers in hot water charging polymerization manner.
These and other objects of the present invention will become apparent from the description hereinafter.